Many different surgical procedures are performed to restore normal function of the musculoskeletal system after acute injury (eg fracture of a bone), or to treat long standing deformities or chronic diseases (eg. arthroplasty to replace arthritic joints). Certain mechanical and spatial parameters define the technical success of these procedures. These parameters typically describe quantities such as:    1. The alignment of bones on each side of a joint or bony fragments fixed in position after a fracture;    2. The shift in the original position of musculoskeletal tissues (e.g. bones or bony fragments, tendons, muscles, and ligaments) with respect to their relative position in the healthy skeleton;    3. The laxity of joints under external distracting or shearing loads; and    4. The relative position of bones during joint motion, including the limits of motion imposed by the joints or body tissues.
Based on extensive experience in reviewing the results of each operative procedure, and the function of the musculoskeletal system in health and disease, orthopedic surgeons have developed quantitative guidelines for target values of each of these parameters. Through reference to these target values, surgeons are able to gauge their success in achieving the “technical goals” of each procedure. Although many surgeons agree on the values of each of the parameters defining the technical success of each operative procedure, few tools are available, during the surgical procedure, to tell the surgeon the extent to which the technical goals of the procedure have been achieved. Numerous disclosures within the patent literature teach methods for guiding surgeons during surgery using computer-based systems within the operating room. These systems have been introduced into many operating rooms in Europe and are generally termed “Surgical Navigation Systems.” These systems generally consist of a computer connected to opto-electrical devices that are utilized to measure the relative position of musculoskeletal structures, typically bones, during the operation. Typically, optical devices are rigidly connected to bony structures and to instruments that are aligned with bony surfaces cut or machined by the surgeon. The system collects information from the measurement devices and is able to calculate the alignment and relative spatial position of each bone and any other feature of interest through reference to the known geometry of each instrument, bone and machined bony surface. Typically, information is displayed in graphical form on a computer monitor to provide information that is useful as a guide to the surgeon. In many systems, the surgeon sees a three-dimensional rendering of the bones of relevance to the procedure and the relative position and alignment of his instruments and reference axes.
Although Surgical Navigation Systems can be useful to the surgeon in providing immediate spatial information during surgery, this approach has several practical shortcomings:    1. To generate accurate, patient-specific models of bony anatomy, computer tomographic (CT) scans are required of each patient. In many parts of the world, this adds significant expense to the use of the System, because of the cost of the CT scan and the time required to prepare a three-dimensional computer model from the CT data. Although this issue may be addressed through use of generic computer models, or through collection of data intraoperatively, both of these solutions involve either a reduction of accuracy or additional expense through time and equipment.    2. Surgical Navigation Systems require additional time and personnel in the operating room to set-up and operate the equipment, to attach optical markers to the skeleton, to register computer models to the optical markers, and to collect and interpret data. This leads to longer operations and a significant reduction in productivity for the operating room. As operating room time is extremely expensive and reimbursement for operative procedures is often fixed, independent of the equipment utilized to perform the procedure, utilization of Surgical Navigation Systems is often commercially unattractive.    3. The computer routines developed for use with these systems are specific to each surgical procedure performed by orthopedic surgeons. This means that surgeons who are not specialized, in that they perform procedures involving different parts of the body (eg. knee replacement, ligamentous reconstruction, and fracture reduction), can only gain access to this technology if they operate at large medical centers with the resources to afford the cost of the Surgical Navigation System and each of the specialized computer programs. As most surgery in the United States, as well as many other countries, is performed at many small facilities, most patients will not be able to receive the benefit of the existing Surgical Navigation Technology.    4. Some of the present Surgical Navigation Systems are cumbersome to use and necessitate increased surgical exposure. This is only possible through larger surgical incisions which increases the length of the patient's recovery and the risk of an intraoperative infection. Some systems also utilize optical marker arrays which are connected to the computer with wires which can complicate the surgical procedure.